We are investigating host- and parasite factors which interact to produce schistosome lesions, through: Purification of schistosome vitellaria granules, and study of their biochemical characteristics; artificial lung granuloma models using schistosome antigen adsorbed onto carrier particles; comparision of schistosome pathogenicity in relation to trematode- and/or host species under controlled conditions; immunofluorescent studies of the localization of schistosome antigens in host tissues and their relationship to pathology; and the effect of drugs, and immunosuppressive measures on these lesions. In addition we shall begin a study on the mechanisms of immunity to schistosomula in mice.